Bug 1447450. Delete WebRenderUserDataTable when its clear. r=kats

FrameProperties have Remove() and Delete() methods. If you call
Remove() you now own the result, therefore we should delete it.

MozReview-Commit-ID: FybBYcbIZIH

--HG--
extra : rebase_source : fe1dc29d5a9bf5bac7b35452d3ad49b8aa0beb6a

gfx/layers/wr/WebRenderCommandBuilder.cpp

@@ -739,6 +739,7 @@ WebRenderCommandBuilder::RemoveUnusedAndResetWebRenderUserData()
 
       if (!userDataTable->Count()) {
         frame->RemoveProperty(WebRenderUserDataProperty::Key());
+        delete userDataTable;
       }
 
       if (data->GetType() == WebRenderUserData::UserDataType::eCanvas) {